New theory says Mars might have had life at one point, but that life didn't evolve quickly enough to help regulate the climate. (AP Photo/NASA, JPL-Caltech, Cornell University, Arizona State University)

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Two astrobiologists have a new explanation for why we haven't found alien life: It didn't evolve quickly enough to keep its planet habitable. "Most early planetary environments are unstable," Aditya Chopra says in a press release. "To produce a habitable planet, life forms need to regulate greenhouse gases, such as water and carbon dioxide, to keep surface temperatures stable." For example, Venus and Mars may have had life at one point, but that life didn't evolve fast enough to keep the planets from heating and cooling past the point of habitability. Or—as Rachel Feltman at the Washington Post puts it—you can blame climate change; only in this case lifeforms didn't enact enough of it.

Chopra and fellow scientist Charley Lineweaver published their "Gaian Bottleneck" hypothesis this week in Astrobiology. The study states extinction is the "cosmic default," and planets "need to be inhabited to remain habitable." "Life on Earth probably played a leading role in stabilizing the planet's climate," Lineweaver says in the press release. Under this model, rapid evolution is more important to the existence of life on other planets than their proximity to a star or the brightness of that star, the study states. But Gizmodo points out Gaian Bottleneck doesn't account for why so-called Gaian regulation is so rare throughout the universe: "Given that there are likely tens of billions of potentially habitable planets in our galaxy…it would have to be one hell of a bottleneck." (But if alien life does exist, it apparently has a soft spot for Florida.)